The invention relates generally to optical fibers and more specifically to optical fibers that maintain the polarization of the incident radiation.
Presently, optical fibers are used as laser devices in a variety of applications. A fiber produces a lasing effect when light introduced into the fiber interacts with a doped core. As light passes through the core it stimulates the dopants and is amplified. The core is typically surrounded by a pure silica inner cladding having a refractive index less than that of the core, and an outer cladding having an index of refraction less than that of the inner cladding. Therefore, the refractive indices of the layers decrease moving from the core to the outer cladding. This profile causes light pumped into the fiber to be internally reflected within the inner cladding.
Laser fibers may be core-pumped or cladding-pumped, depending on where the source light is introduced. In the latter case, light directed into the cladding (e.g., from the side of the fiber) is reflected into the core to cause lasing. To increase the amount of light that interacts with the core, thereby increasing pumped-light absorption efficiency, the inner cladding may be polygonally (as opposed to circularly) shaped. The non-circular inner cladding causes ray distortion and mode mixing of the incident light, thereby causing the rays to interact with the core more frequently than would be the case in a circular configuration. A circular inner cladding causes light to be continuously reflected in a helical path within the circular cladding and aroundxe2x80x94rather than intoxe2x80x94the core.
In some applications, it is important to preserve the polarization of the incident light. For example, various devices (e.g., Raman amplifiers pumped by multiple lasers) require a polarized light signal, so fiber conducting light to such a device should retain the original polarization of the source signal.
Single-mode polarization-preserving fibers generally rely on asymmetrical features of the fiber to maintain the polarization of the input light. In these fibers, two orthogonally polarized modes propagate in the fiber, and the asymmetry of the fiber maintains their polarization. One example of this asymmetry, illustrated in FIG. 1, utilizes an elliptically shaped core 160 disposed within a circular cladding 170. An alternative approach to maintaining polarization is to induce birefringence in the fiber by stressing the cladding in one direction. The component polarized in the direction in which stress is induced is slowed to the point of being eliminated, while the other component is allowed to propagate through the fiber. For example, a linearly polarized incident light decomposes into both x and y polarizations as it propagates down the fiber. Stressing the fiber in the x direction will slow one of the components to the point of virtual elimination. Therefore, the polarization of the incident light is preserved at the end of the fiber.
The present invention relates to preserving the polarization characteristics of the incident light and increasing the pumped-radiation absorption efficiencyxe2x80x94the amount of absorbed light that interacts with the corexe2x80x94by controlling the shape of the cladding and stress members and their positions relative to each other and the core.
In one aspect, the invention relates to an optical fiber including a lasing core that carries radiation. The core has index of refraction nc. Additionally, the fiber includes a primary cladding with an index of refraction npc that surrounds the core, a secondary cladding with an index of refraction nsc, that surrounds the primary cladding. Within the primary cladding a pair of stress members, each with an index of refraction nsm, are disposed on opposite sides of the core. The relationships among the indices of refraction are as follows: nc greater than npc greater than nsc and npc greater than nsm. Additionally, the stress members have a coefficient of thermal expansion different from that of the primary cladding. This difference induces birefringence within the fiber. Each of the stress members has a flat surface facing the core and the flat surface of the other stress member. Furthermore, the primary cladding has a pair of opposed flat surfaces substantially perpendicular to the stress-member flat surfaces. The stress-member flat surfaces and the primary cladding flat surfaces cooperate to reflect light into the core as it propagates through the fiber.
In one embodiment, the stress-member flat surfaces have a length at least equal to the core diameter. In a further embodiment, the cladding flat surfaces have a width dimension at least equal to the core diameter. In yet a further embodiment, the stress-member flat surfaces are separated by a distance, and the cladding flat surfaces at least span this distance. The stress members may be solid or fluid (i.e., a liquid, gas, or gel).
In another aspect, the present invention relates to a method of maintaining polarization and improving pump-energy efficiency including the step of providing an optical fiber that has a lasing core, a non-circular primary cladding, a secondary cladding, and a pair of stress members disposed within the primary cladding on opposite sides of the core. The primary cladding and the stress members have different coefficients of thermal expansion, thereby resulting in birefringence within the primary cladding. Additionally, the method includes the step of radially pumping polarized light into the fiber. The birefringence within the primary cladding preserves the polarization of the pumped light, and the non-circular cladding directs the light at the core as the light propagates down the fiber.
In one embodiment, each of the stress members has a flat surface that faces the core. In a further embodiment, the primary cladding has a pair of opposed flat surfaces that are substantially perpendicular to the stress-member flat surfaces, thereby cooperating to direct pumped light at the core. In yet a further embodiment, the core has an index of refraction nc, the primary cladding has an index of refraction npc, the secondary cladding has an index of refraction nsc, and the stress members have an index of refraction of nsm. The relationships among the indices of refraction are nc greater than npc greater than nsc and npc greater than nsm.